1. Field of Use
This invention relates generally to keyboards for electrical devices such as telephones, computers, calculators or the like.
In particular, it relates to keyboards comprising a dielectric panel having manually operable keys thereon or extending therethrough.
2. Description of the Prior Art
Some electrical devices can be adversely affected or will malfunction if a high voltage charge of static electricity is applied thereto, as when a human operator reaches out to manipulate the keys on the keyboard of the device. Typically, a static charge can build up as the operator moves about the room and can become quite high (on the order of 20,000 volts). Low humidity, carpeting and the operator's clothing contribute to the effect. The static charge can travel directly to the keyboard from the operator's fingers, from an object being held, such as a pencil, or from the operator's clothing, if it comes near the keyboard. Unless precautions are taken, the static charge travels through or around the keyboard panel to electrical components within the device, such as solid state devices, or electrical conductors connected to devices located within or remote from the device. In a phone system, for example, this can cause relays or circuit breakers to trip and results in a great deal of down time until they can be reset.
One commonly used method to avoid this problem is grounding the charge before it reaches the device. Thus, some devices are located on a grounding mat on a desk or table, or a large floor mat is provided at the operator's work station. The grounding mat bleeds off the static charge before it can be transferred to the device. However, such mats are relatively large, costly and can be unintentionally removed or displaced or otherwise rendered ineffective.
Another known method to avoid the problem is to employ a dielectric keyboard panel on the device to prevent the charge from reaching sensitive components. Thus, in some keyboards the keys are located on or project through a panel made of electrical insulating material such as plastic. Sometimes the panel takes the form of a flat transparent electrically non-conductive plastic sheet which has ink applied thereto in desired patterns. In some cases a layer of ink comprising electrically non-conductive clear plastic in which electrically conductive metal particles are suspended is applied to the plastic sheet to give the panel a metallic or flaked metal appearance. Such an ink layer is formulated of a mixture of very fine metal particles (such as aluminum flakes or powder) and a hardenable liquid plastic. The mixture is applied to the flat plastic sheet as a thin coating by brushing, spraying, printing or screening, and is then dried. However, prior art mixtures have not been entirely satisfactory because of the tendency of the metal particles to be non-uniformly suspended or to settle out or to clump together while the plastic is still in liquid form and during drying. Non-during drying. Non-uniform suspension of the electrically conductive particles substantially reduces the dielectric properties and effectiveness of the panel. More specifically, if the particles settle out or rise to the surface of the liquid plastic or cling to each other, as they tend to do when a conventional wetting agent or flow promoter is added to the mixture to facilitate application to the plastic sheet, they form an electrically conductive layer. However, it is extremely important that the coating be electrically non-conductive inasmuch as, when low-humidity conditions exist, as they do much of the time in northern climates, a static discharge will be conducted through the conductive ink layer or coating and into the associated electrical device. In a phone system, for example, this can cause relays or circuit breakers to trip and result in a great deal of down time until they can be reset. A high voltage static charge can flash over or flow through or around the coated panel and to the dvice with adverse affects.